Study Of Unconventional Superconductivity In Fe_1+yTe

One hundred years after its discovery by Heike Kamerlingh Onnes,superconductivity is still one of the most fascinating and challenging topics in condensed-matter physics.From the initial mercury superconductor to copper-based superconductors that breaks the Mc Millan limit,as well as recently discovered iron-based superconductors,the study on the superconducting mechanism and their applications has greatly promoted the development of condensed matter physics.Compared to copper-based superconductors,the electron correlation in the iron-based superconductors is relatively weak.Due to the similarity of phase diagram,lattice structure,and unconventional superconducting pairing symmetry between copper-and Iron-based superconductors,it will shed lights on understanding the high-T_csuperconductivity if we can clarify the iron-based superconductivity,and it will contribute to the search for new superconductors with higher T_c,even for the room-temperature superconductors.In the iron-based superconducting family,the 11-system（such as Fe Se,Fe Te,etc.）has the simplest structure,and its high upper critical magnetic field indicates the possibility of unconventional pairing mechanism.Therefore,the 11-system has attracted widespread attention.Recent studies have shown the existence of topological surface states in Fe Se Te system,which provides an ideal platform for searching for intrinsic topological superconductors by combing the topology and superconductivity in one material.Additionally,the magnetism and superconductivity can be tuned by changing the Fe content of the sample,implying there is a competition between magnetism and superconductivity in the system.Motivated by the above facts,this thesis mainly investigates the anomalous upper critical field,vortex flow behavior,and reentrant superconductivity by measuring the low-temperature transport properties on high-quality Fe_1+yTe thin film samples prepared by chemical vapor deposition（CVD）.The thesis is composed of the following five chapters:The first chapter firstly introduces the basic knowledge of iron-based superconductivity,then the topological superconducting states appeared in the 11-system of iron-based superconductors and the correlation between superconductivity and magnetism were mainly reviewed.Chapter 2 introduces the experimental methods and instruments used in this thesis,including the methods of sample growth and basic characterizations,the working principle of low-temperature transport measurement equipment,and the micro/nano processing methods,as well as some detailed device fabrication process.Chapter 3 studies the upper critical field behavior and vortex creep behavior of Fe_1+yTe.By measuring the resistance temperature curves under different magnetic fields,we found that there is anomalous upper critical field（exceeding the Pauli limit）in the sample,which may be related to impurity scattering caused by excessive Fe in the sample.For samples with significantly broadened superconducting transition under different magnetic field,systematic analysis shows that as the temperature increases,the vortex creep transitions from two-dimension（2D）to three-dimension（3D）.Chapter 4 is the main section of the thesis.It mainly studies the influence of Fe content on the transport behavior of Fe_1+yTe samples.The exotic superconducting reentrant behavior were observed.More importantly,the transport measurements revealed an anomalous temperature dependent critical field behavior of second superconducting phase.The experimental results were analyzed and discussed.A preliminary summary is presented in Chapter 5 together with an outlook on future research.